(1) Field of the Invention
The present invention relates to a hybrid positioning method and device.
(2) Description of Related Art
The technical field of the invention is that of manufacturing navigation systems mounted on board helicopters.
The present invention relates in particular to a navigation and/or positioning method and device in which inertial measurements are combined with data relating to the distance ranges between a moving body carrying the device (in particular a rotorcraft) and satellites.
In the meaning of the present application, and without explicit or implicit indication to the contrary, the terms “measurement”, “data”, “signal”, and their derivatives are considered as being equivalent, and the verbs “combining”, “hybridizing”, “coupling”, as well as their derivatives are also considered as being equivalent.
Generally speaking, a GPS receiver comprises four main blocks or modules:                a first module is an HF head (also known as an “RF front end”) which pre amplifies signals picked up by the antenna of the GPS receiver, and implements a frequency change;        a second module is a clock supplying the other modules with an accurate time and frequency reference; since the accuracy of the clock is not absolute, its time and frequency errors are modeled by means of biases;        a third module is a signal processor module that decodes navigation messages coming from satellites, to supply the ephemerides of the constellation of 24 satellites; this first type of data corresponds to the positions measurements of 24 satellites (or three-dimensional vectors,{right arrow over (p)}SVi=[xSViySVizSVi]for i=1 to 24); this constitutes data that is needed for calculating these 24 positions, assuming that time is known accurately; this module further measures the propagation times of the signals that have been both received, and coming from those satellites that are in view. The number of such signals or the number of channels (NCH) can vary. The calculation of these NHC/distances corresponds to the respective measured propagation durations; the common bias that affects these measurements corresponds to the offset of the clock and can be equivalent to several thousands of kilometers; the ranges as determined in this way are generally referred to as “pseudo-ranges”; in the present application, and unless explicitly or implicitly indicated to the contrary, the terms “range” and “pseudo-range” are used interchangeably to designate distance range data delivered by this receiver module; some receivers also deliver measurements of the derivative in this distance values (called “pseudo-range” rates); and        a fourth module is “calculating the navigation solution” and solves the following system of equations:ri2=(xi−x)2+(yi−y)2+(zi−z)2+Ct for i=1 to NCH and in which:{right arrow over (p)}=[xyz]T is the position of the antenna, t being the offset of the clock, and C being the speed of light; since there are four unknowns (x, y, z, t), solving the system requires four scalar equations; when the number of receiver channels exceeds four, the overabundant data is used to reduce error and to increase integrity, e.g. by using the least squares method and by detecting points that depart from the general trend.        
This block thus delivers the position and the time of the receiver corrected for its bias, and where appropriate a measurement of the derivative of the position of the moving body, i.e. the velocity of the carrier.
It is known that such range data relative to satellites can be combined with inertial data delivered by an inertial navigation unit on board the moving body, by using a Kalman filter so as to determine the position of the moving body, e.g. as described in the document “GPS and INS integration with Kalman filtering for direct geo-referencing of airborne imagery”, ETH Hönggerberg, Zurich, Institute of Geodesy and Photogrammetry.
The meanings of the usual abbreviations as used below are summarized as follows:
AAIM: aircraft autonomous integrity monitoring;
AFCS: automatic flight control system;
ARAS: aircraft based augmentation system;
EGNOS: European geostationary navigation overlay system;
FIFO: first-in, first-out;
FMS: flight management system;
GBAS: ground based augmentation system;
GNSS: global navigation satellite system;
GPS: global positioning system;
INS: inertial navigation system;
PVT: position, velocity, time;
RAIM: receiver autonomous integrity monitoring;
SEAS: spaced-based augmentation system;
TTA: time to alert;
VCO: voltage controlled oscillator;
WAAS: wide area augmentation system.